The present invention relates to conditioning of a polishing pad employed in chemical-mechanical polishing (CMP). More particularly, the present invention relates to a conditioning wafer and an automatic polishing pad conditioning process using the conditioning wafer.
Chemical mechanical polishing (sometimes referred to as "CMP") typically involves mounting a wafer faced down on a holder and rotating the wafer face against a polishing pad mounted on a platen, which in turn is rotating or is in an orbital state. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. In semiconductor wafer fabrication, this technique is commonly applied to planarize various wafer layers such as dielectric layers, metallization layers, etc.
Unfortunately after polishing on the same polishing pad over a period of time, the polishing pad suffers from "pad glazing." As is well known in the art, pad glazing results when the particles eroded from the wafer surface along with the abrasives in the slurry tend to glaze or accumulate over the polishing pad. Pad glazing is particularly pronounced during CMP an oxide layer such as a silicon dioxide layer (hereinafter referred to as "oxide CMP"). By way of example, during oxide CMP, eroded silicon dioxide particulate residue along with the abrasives in the slurry glaze the polishing pad. Pad glazing is undesirable because it reduces the polishing rate of the wafer surface and produces a non-uniformly polished wafer surface due to uneven removal of the glaze, e.g., the peripheral region of the wafer surface may not be polished to the same extent as the center region of the wafer surface or vice-versa.
One way of achieving and maintaining a high and stable polishing rate is by conditioning the polishing pad on a regular basis, i.e. either every time after a wafer has been polished or simultaneously during wafer CMP. FIG. 1A shows a top view of a conditioning sub-assembly 100 including a polishing pad 102. Conditioning sub-assembly 100 also includes a conditioning arm 104 that is disposed above polishing pad 102 and capable of pivoting about a pivoting point 106. Conditioning arm 104, as shown in FIG. 1A, is typically longer in length than a diameter of the polishing pad. For illustration purposes, FIG. 1B shows a bottom view of conditioning arm 104 of FIG. 1A. The bottom surface of conditioning arm 104 includes a plurality of diamond abrasive particles 108, which are almost uniformly arranged on the conditioning arm such that if conditioning arm 104 contacts polishing pad 102, abrasive particles 108 engage with a substantial portion of the polishing pad.
Before conditioning sub-assembly 100 of FIG. 1A begins conditioning of polishing pad 102 (the process of conditioning a polishing pad is hereinafter referred to as "pad conditioning"), conditioning arm 104 is lowered automatically to contact a polishing pad 102, which may be rotating or in orbital state. A pneumatic cylinder (not shown to simplify illustration) may then apply a downward force on conditioning arm 104 such that abrasive particles 108 contact and engage with a substantial portion of polishing pad 102. During pad conditioning, conditioning arm 104 pivots on pivoting end 106 and sweeps back and forth across polishing pad 102 like a "windshield wiper blade" from a first position 104' (shown by dashed lines) at one end of the polishing pad to a second position 104" (shown by dashed lines) at the other end of the polishing pad. This mechanical action of conditioning arm 104 allows abrasive particles 108 to break up and remove the glazed or accumulated particles coated on the polishing pad surface.
Furthermore, the mechanical action of conditioning arm 104 also facilitates the formation of grooves or perforations on polishing pad 102. Although polishing pad 102 can be provided with grooves or perforations for slurry distribution and improved pad-wafer contact, the effectiveness of such grooves is reduced over time due to normal polishing. The pad conditioning process thus serves to reintroduce grooves or roughen the pad surface. Grooves produced during pad conditioning facilitate the polishing process by creating point contacts between the wafer surface and the pad, increase the pad roughness and allow more slurry to be applied to the substrate per unit area. Accordingly, the grooves generated on a polishing pad during conditioning increase and stabilize the wafer polishing rate.
Unfortunately, the current pad conditioning process suffers from several drawbacks. By way of example, the sweeping action of the conditioning arm across the polishing pad distributes the mechanical action of the abrasive particles almost uniformly throughout the polishing pad surface, without any particular attention to that part of the polishing pad where the wafer specifically undergoes CMP. The part of the polishing pad that actually contacts the wafer during CMP, however, may need concentrated conditioning that is not provided by the sweeping action of the conditioning arm. By way of example, in a CMP apparatuses AvantGaard 676 and 776, commercially available from Integrated Process Equipment Corporation, of Phoenix, Ariz., the center region of the polishing pad, which actually contacts the wafer during CMP, needs concentrated conditioning. Therefore, according to conventional conditioning processes, parts of the polishing pad, which normally contact the wafer during CMP, are not effectively conditioned leading to lower material removal rates and non-uniform polishing of the wafer surface. Furthermore, over conditioning of the pad to compensate also shortens the life of the polishing pad, which are expensive. In a typical wafer fabrication facility, where several CMP apparatus are employed, the replacement cost of polishing pads can be a significant expense.
What is therefore needed is an improved apparatus and process for pad conditioning that provides higher material removal rates and uniform polishing of the wafer surface.